Process for the preparation of methionine

ABSTRACT

The invention relates to a process for the preparation of D,L-methionine, in which carbon dioxide is fed to an aqueous potassium methioninate solution obtained by hydrolysis of 5-(2-methylmercaptoethyl)hydantoin, in order to precipitate out crude methionine, which is separated off and purified, where, for the purposes of purification, an aqueous solution of the separated-off crude methionine is prepared and subjected to a recrystallization, characterized in that the solution from which the recrystallization takes place contains potassium ions and also a crystallization additive, where the crystallization additive is a nonionic or anionic surfactant, or a mixture of different nonionic or anionic surfactants, and that the recrystallization takes place by introducing a 60 to 110° C.-hot methionine solution into a 35 to 80° C.-warm methionine suspension, the temperature of which is lower than that of the introduced solution, the temperature of the methionine suspension being maintained between 35 and 80° C. during the addition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of PCT/EP2013/053795 filed on Feb.26, 2013. This application is based upon and claims the benefit ofpriority to European Application No. 12160257.7 filed on Mar. 20, 2012.

BACKGROUND OF THE INVENTION

The invention relates to a process for the preparation of D,L-methioninewith a high bulk density, where the methionine is purified byrecrystallization.

L-Methionine is an essential amino acid which is of great industrialimportance as a feed supplement. Since D- and L-methionine are ofidentical nutritional value, the racemate is usually used as feedsupplement. The synthesis of D,L-methionine proceeds starting frommethylmercaptopropionaldehyde and hydrogen cyanide with the preparationof the intermediate 5-(2-methylmercaptoethyl)hydantoin, which can beconverted to the methioninate by hydrolysis.

Various processes are known both for the hydrolysis of hydantoin andalso for the subsequent release of methionine from its salt. The presentinvention relates to the preparation of methionine by the so-calledpotassium carbonate process, which is described for example in EP 1 256571 A1 and DE 19 06 405 A1. In this connection,5-(2-methylmercaptoethyl)hydantoin in aqueous solution is firstlyreacted with potassium carbonate to give potassium methioninate with therelease of carbon dioxide and ammonia. By introducing carbon dioxide,the basic potassium methioninate solution is neutralized and methionineis precipitated out. The crude methionine obtained in this way, however,is produced in the form of platelet-like or flake-like, poorlyfilterable crystals, which are shown in FIG. 1.

To control the foaming and to improve the crystal quality, the crudemethionine precipitation according to EP 1 256 571 A1 takes place in thepresence of an antifoam. This process has the disadvantage thatmethionine is obtained in the form of spherical, but porous particles,which are shown in FIG. 2. Because of its porous structure, themethionine obtained in such a way has to be washed with large amounts ofwater and dried, incurring high energy costs, in order to arrive at amarketable product.

The addition of additives during the crude methionine precipitation canimprove the crystal quality. As additives, for example sorbitan laurate,polyvinyl alcohol, hydroxypropylmethylcellulose, gluten or casein areknown from JP 11158140 and JP 10306071. According to these processes,methionine crystals with a bulk density of up to 770 g/l are obtained.It has proven to be disadvantageous for these processes that they arecarried out as batch processes or in merely semicontinuous mode.

It is likewise known to improve purity and bulk density of methionine byrecrystallization of crude methionine. JP 2004-292324 discloses, forexample, the recrystallization of crude methionine by adding polyvinylalcohol or gluten, giving pure methionine with a bulk density of up to580 g/l. The recrystallization takes place by the dropwise addition of ahot methionine solution to a cold methionine suspension, with methionineprecipitating out as a result of cooling the hot solution. Adisadvantage has again proven to be that this process is not carried outcontinuously.

EP 1 451 139 A1 describes the recrystallization of methionine in thepresence of hydroxyethylcellulose, with initially methionine crystalshaving a bulk density of up to 620 g/l being obtained. In this case, adisadvantage has proven to be that in a continuous recrystallizationprocess there is an accumulation of the continuously added additive as aresult of reusing the filtrate for dissolving crude methionine and thatan increasing additive concentration results in a reduction in the bulkdensity. For this reason, hydroxyethylcellulose is not advantageous foruse as crystallization additive in a continuous process in which thefiltrate of the pure methionine is reused for dissolving crudemethionine. The reuse of the recrystallization filtrate is of decisiveimportance for the economic feasibility of the process on an industrialscale since losses of dissolved methionine are avoided and thegeneration of wastewater is minimized.

JP 46 019610 B1 describes a process for the recrystallization ofmethionine, which however does not allow to achieve high bulk densitiesfor methionine.

It is an object of the present invention to provide a process for thepreparation of methionine which avoids the described disadvantages. Themethionine obtained by the process should be readily filterable and havea high bulk density. Furthermore, the process should be able to becarried out in continuous mode and in particular should avoid thenegative consequences of accumulation processes.

BRIEF SUMMARY OF THE INVENTION

To achieve this object, the present invention provides a process for thepreparation of D,L-methionine, in which carbon dioxide is fed to anaqueous potassium methioninate solution obtained by hydrolysis of5-(2-methylmercaptoethyl)hydantoin, in order to precipitate out crudemethionine, which is separated off and purified, where, for the purposesof purification, an aqueous solution of the separated-off crudemethionine is prepared and subjected to a recrystallization. In theprocess, the solution from which the recrystallization takes placecontains potassium ions and also a crystallization additive, where thecrystallization additive is a nonionic or anionic surfactant, or amixture of different nonionic or anionic surfactants. According to theinvention, the recrystallization takes place by introducing a 60 to 110°C.-hot methionine solution into a 35 to 80° C.-warm methioninesuspension, the temperature of which is lower than that of theintroduced solution, the temperature of the methionine suspension beingmaintained between 35 and 80° C. during the addition.

The hot methionine solution is preferably cooled rapidly by introducingit into the initial charge of cooler methionine suspension, as a resultof which a superconcentration of dissolved methionine is produced andmethionine precipitates out from the solution. In this way, thepreference in the crystal growth spatial direction is interrupted and anisometric crystal habit is achieved. However, besides the desiredisometric crystals, it is also possible for undesired new, platelet-likecrystal germs to form as a result of this rapid cooling mode. In onepreferred embodiment of the process according to the invention, thesecan be specifically redissolved by moderately increasing the temperatureby 5-15° C., preferably by 6-12° C., compared to the mixing temperature.

As a result of the combination according to the invention of thepresence of potassium ions, the addition of crystallization additive andthe temperature control of the recrystallization, coarsely granular,readily filterable methionine crystals with a bulk density of above 500g/l are obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts crude methionine in the form of platelet-like orflake-like, poorly filterable crystals;

FIG. 2 depicts methionine in the form of spherical, but porousparticles;

FIG. 3 depicts methionine obtained without the addition ofcrystallization additives, without the presence of potassium by simplecooling;

FIG. 4 depicts pure methionine as obtained by one embodiment of thepresent disclosure;

FIG. 5 depicts a schematic of a two-stage recrystallization processaccording to one embodiment of the present disclosure; and

FIG. 6 is a graph showing a temperature-dependent solubility behavior ofmethionine.

DETAILED DESCRIPTION OF THE INVENTION

In a preferred embodiment of the process, the crystallization additiveis one of the compounds shown in formulae 1 to 3, or a mixture thereof:R¹—O—SO₃M  (formula 1)R²—O—(CH₂)_(n)—SO₃M  (formula 2)R³—(O—C₂H₄)_(n)—O—SO₃M  (formula 3)where n is an integer from 1 to 12, M is sodium or potassium and R¹, R²and R³ are a linear, branched or cyclic, saturated or unsaturated C₈ toC₂₀ alkyl group or an aryl group.

In a preferred embodiment of the aforementioned compounds, n=2 and R¹,R² and R³ are linear, saturated C₈ to C₁₈ alkyl groups.

In a further embodiment of the process, the crystallization additive isa sorbitan fatty acid ester or a mixture of different sorbitan fattyacid esters, preferably polyethoxylated sorbitan fatty acid esters. In aparticularly preferred embodiment, the crystallization additive is apolyethoxylated sorbitan stearate, and in particular a polyethoxylatedsorbitan tristearate according to formula 4:

where w+x+y+z=20.

The concentration of the crystallization additive in the solution fromwhich the recrystallization takes place is preferably at least 50 ppmbased on the total mass of the solution, particularly preferably atleast 100 ppm, most preferably at least 400 ppm. In order to achieve anoptimum dosing and distribution of the crystallization additive, it ispreferably used in the form of an aqueous solution or emulsion, in whichcase the concentration of the crystallization additive in the solutionor emulsion is preferably 2 to 15% by weight.

In a preferred embodiment of the process according to the invention, thesolution from which the recrystallization takes place additionallycomprises an antifoam. The antifoam has the function of suppressing thefoam which is formed when handling the methionine solution andsuspension and which is intensified and/or caused by some of theaforementioned crystallization additives. Moreover, a synergistic effectarises for the attained bulk densities of methionine when simultaneouslyusing antifoam and crystallization additives, as a result of which bulkdensities of more than 600 g/l are achieved, the negative consequencesof accumulation processes are simultaneously avoided and the processaccording to the invention can thus also be carried out in continuousmode.

The antifoam preferably comprises silicone oil, preference being givento using a silicone oil with a kinematic viscosity of 0.65 to 10 000mm²/s (measured at 25° C. in accordance with DIN 53018), particularlypreferably from 90 to 1500 mm²/s. The antifoam can further containconstituents which are effective as emulsifiers, for example mixtures ofpolyethoxylated fatty acids and polyethoxylated fatty alcohols. Theantifoam can likewise comprise silica. In a preferred embodiment, theantifoam is an aqueous solution which comprises 5 to 10% by weight ofsilicone oil, 0.05 to 1% by weight of silica, 0.5 to 5% by weight of amixture of polyethoxylated fatty acids, and 2 to 7% by weight of amixture of polyethoxylated fatty alcohols. Preferably, the antifoam isused in a mixture with the crystallization additive, the crystallizationadditive being admixed in a concentration of preferably 2 to 15% byweight. In order to achieve a continuous, stable dosing of the antifoam,it is preferably further diluted with water prior to being used.

The use of silicone oil antifoams leads to silicon being able to bedetected in the methionine prepared by the process according to theinvention using a suitable analysis method (e.g. X-ray photoelectronspectroscopy, abbreviated to XPS). Therefore, a further object of thepresent invention is D,L-methionine obtained by the process according tothe present invention, wherein a silicone oil antifoam is used in saidprocess.

Surprisingly, it has been found that the presence of potassium ions inthe solution from which the recrystallization takes place is importantfor the crystallization success. Preferably, the potassium ionconcentration in the solution from which the recrystallization takesplace is 1 to 30 g/kg, particularly preferably 2 to 14 g/kg, mostpreferably 5 to 10 g/kg. The potassium preferably passes into therecrystallization solution with the crude methionine. The potassiumconcentration can be adjusted for example by introducing washing waterduring the crude methionine filtration and/or by introducing freshwaterto the pure filtrate used for dissolving the crude methionine and/or byintroducing potassium into the pure filtrate used for dissolving thecrude methionine.

According to the invention, the crude methionine is dissolved in anaqueous solution before the recrystallization. This is effectedpreferably by heating the solution to a temperature of at least 95° C.,particularly preferably by heating to boiling temperature. To dissolvethe crude methionine, it is possible to use, for example, freshwater,the filtrate of the pure methionine, or the condensate of the vacuumcrystallization described below or mixtures thereof.

According to the invention, crystallization additive and the antifoamare added to the aqueous matrix used for dissolving the crudemethionine. In one possible embodiment of the process, thecrystallization additive and the antifoam are also added to the solutionfrom which the crude methionine is precipitated out.

Preferably, the recrystallization takes place by introducing an 85 to110° C.-hot crude methionine solution into a 35 to 60° C.-warmmethionine suspension, the temperature of the mixture that is formed asa result being kept constant between 35 and 60° C. In this connection,the volume ratio of the introduced crude methionine solution to theinitial charge of methionine suspension is preferably in the range from1:1 to 1:10, particularly preferably from 1:3 to 1:6.

In a further preferred embodiment of the process, the recrystallizationis carried out in two stages. For this, in the first recrystallizationstage, an 85 to 110° C.-hot crude methionine solution is introduced intoa 60 to 80° C.-warm methionine suspension and the temperature of themixture that is formed as a result is kept constant between 60 and 80°C. It is particularly preferred here to remove some of the methioninesuspension from the first recrystallization stage and to return it againto the recrystallization via a circulation circuit, the temperature ofthe suspension in the circulation circuit being increased by 6 to 12° C.The 60 to 80° C.-warm methionine suspension obtained in the firstrecrystallization stage is introduced, in a second recrystallizationstage, into a 35 to 60° C.-warm methionine suspension, the temperatureof the mixture that is formed as a result being kept constant between 35and 60° C. The volume ratio of the introduced methionine suspension tothe initial charge of methionine suspension is preferably in the rangefrom 1:1 to 1:10, particularly preferably from 1:3 to 1:6.

Besides a first or a first and second recrystallization stage, theprocess according to the invention can also involve furtherrecrystallization stages.

In the event of a multistage procedure, all stages can be charged inparallel with crude methionine at the same temperature differencebetween crude methionine solution and initial charge of methioninesuspension. The multistage recrystallization can also be carried outsuch that the recrystallization stages are successively charged with themethionine solution from the proceeding stage, the temperaturedifference between crude methionine and methionine solution beingselected such that the methionine solution from one recrystallizationstage can be used as crude methionine for the next recrystallizationstage. This has the advantage of reduced formation of undesiredplatelet-like crystals as a result of excessively large temperaturedifferences. The multistage recrystallization of course also involvesmixed forms of parallel and consecutive charging of therecrystallization units.

The preferred temperature control for the process according to theinvention arises from the temperature-dependent solubility behaviour ofmethionine shown in FIG. 6.

In economic terms, it is expedient to cool the methionine solutions toan end temperature of from 30 to 50° C. since, in so doing, both theamount of methionine remaining in solution can be minimized, and alsothe use of expensive cooling media for the purposes of further coolingthe methionine-containing solutions is avoided.

In a preferred embodiment of the process, the recrystallization iscarried out by vacuum crystallization. Here, the pressure in the firstrecrystallization stage is preferably 100 to 1000 mbar, particularlypreferably 150 to 400 mbar. If a two-stage recrystallization is carriedout, the pressure in the second recrystallization stage is preferably 35to 200 mbar, particularly preferably 35 to 100 mbar. Preferably, thewater evaporated in the vacuum crystallization is condensed and isreused for dissolving further crude methionine.

In one preferred embodiment of the process, some of the methioninesuspension is removed from the first and/or one of the otherrecrystallization stages and is returned again via a circulationcircuit. In the first crystallization stage, the hot methionine solutionis preferably added to the circulated colder suspension in a volumeratio of 1:3 to 1:6. Upon this rapid cooling, a high supersaturation isproduced, as a result of which, on the one hand, relatively largecrystals grow isometrically, or else new, small, platelet-like crystalsare formed. The small platelet-like crystals are also dissolved again inthe recirculation line by increasing the temperature by 6 to 12° C., theisometric relatively large crystals being retained.

Separating off the pure methionine from the mother liquor of therecrystallization preferably takes place by filtration, for examplepressure or vacuum filtration, or by means of centrifuges, for exampletrailing-blade, pusher-type or screen centrifuges.

The process according to the invention can either be carried outcontinuously or else discontinuously or semicontinuously.

The attached FIGS. 1 to 4 show electron micrographs of crystallinemethionine. FIG. 1 shows crude methionine as is obtained from the crudemethionine precipitation without the addition of crystallizationadditives. FIG. 2 shows crude methionine from the crude methionineprecipitation with the addition of an antifoam according to EP 1 256 571A1. FIG. 3 shows methionine as is obtained without the addition ofcrystallization additives, without the presence of potassium by simplecooling. FIG. 4 shows pure methionine as is obtained with the processaccording to the invention.

FIG. 5 shows, by way of example and in diagrammatic form, an arrangementfor carrying out the process according to the invention in a preferredtwo-stage recrystallization. In container A, crude methionine isdissolved with an aqueous matrix, which can comprise the filtrate of thepure methionine, at a temperature of from 90 to 100° C. The temperatureis adjusted via a circulation pump and an external heat exchanger. Thecrystallization additive according to the invention including antifoamis added continuously to the aqueous matrix. The methionine solution isheated to 100 to 110° C. via one or more heat exchangers B and then fedto the circulation circuit of the first vacuum crystallizer D. Thecirculated suspension has a temperature of 60 to 70° C. The ratio ofamount fed in to circulation amount is in the range from 1:3 to 1:6. Theaverage residence time of the mixture in the circulation circuit is 5 to15 sec. The mixture is heated to 65 to 75° C. via a heat exchanger C, asa result of which fine and in particular platelet-like methioninecrystals rapidly dissolve because of their relatively large specificsurface area. The mixture then passes to the first vacuum crystallizerD, in the top region of which, at a pressure of 180 to 200 mbar, waterevaporation and cooling of the mixture occurs. This results incrystallization of dissolved methionine. The methionine crystals settleout in the vacuum crystallizer at differing rates. Small, platelet-likecrystals settle out more slowly than coarse, isometric crystals. Thesuspension for recirculation is removed in the upper region of thevacuum crystallizer, where predominantly smaller, platelet-like crystalsare found on account of the slower settling rate. The coarse, isometriccrystals are removed in the lower region of the vacuum crystallizer Dand fed to the circulation circuit of the second vacuum crystallizer E.The suspension circulated here has a temperature of 30 to 50° C. Theratio of amount fed in to circulation amount is in the range from 1:3 to1:6. The pressure in the vacuum crystallizer E is 60 to 80 mbar. Invacuum crystallizer E, further methionine is crystallized, as a resultof which the average particle size of the methionine crystals inparticular is increased. If required, the methionine suspension can bepassed to an interim container F in order to permit a postprecipitationof methionine. Finally, the methionine is isolated in a suitablesolid/liquid separation step G, where the filtrate obtained can, ifrequired, be returned to container A.

The examples below aim to explain the invention in more detail.

EXAMPLES Example 1

Recrystallization in the Presence of a Crystallization AdditiveAccording to the Invention Compared with a Known CrystallizationAdditive

60 g of methionine, 305 g of water and 35 g of crude methionine filtratewere introduced into a flask and circulated via a heat exchanger bypumping at a temperature of 40° C. As a result of the potassiumcarbonate present in the crude methionine filtrate, the potassium ionconcentration was ca. 7 g/kg. A solution, heated to 90° C., of 150 g ofmethionine in 990.5 ml of water and 109.5 g of crude methionine filtratewas added to this suspension at a rate of 18 ml/min, during which thetemperature of the initial charge of suspension was kept at 40° C. Afteradding 650 ml of the hot solution, 500 ml of suspension were removed andthen a further 500 ml of the hot solution were metered in at a rate of18 ml/min. The resulting suspension was discharged, the amount of foamwas determined, and the methionine was filtered off and washed with 300ml of acetone. After drying the methionine, the bulk density wasdetermined.

The recrystallization experiments were carried out in the presence ofthe following additives, the stated concentration being established byadding the additive to both starting solutions/suspensions. Theconcentration data give the total active ingredient content of theadditive without water based on the total mass of the solution orsuspension. Additive 1 was an aqueous mixture of antifoam andcrystallization additive according to EP 1 451 139 A1, consisting of 2%by weight of hydroxyethylcellulose and 2% by weight of a polyethoxylatedfatty acid (C₁₈H₃₇—(CO)—O—(CH₂—CH₂—O)₇—H). Additive 2 was an aqueousmixture of a crystallization additive and an antifoam compositionaccording to the present invention, consisting of 6.1% by weight ofsilicone oil with a kinematic viscosity of 1000 mm²/s (AK 1000,Wacker-Chemie GmbH), 0.25% by weight of hydrophobicized silica (SipernatD10, Evonik Degussa GmbH), 2.6% by weight of a polyethoxylated fattyacid mixture (Intrasol® FS 18/90/7, Ashland Deutschland GmbH), 3.7% byweight of a polyethoxylated fatty alcohol mixture (2.35% by weight ofMarlipal®, Sasol Germany GmbH, 1.35% by weight of Brij C2, CrodaChemicals Europe) and 5.1% by weight of a fatty alcohol sulphate(Sulfopon® 1218 G, Oleochemicals) according to the formula:C_(n)H_(2n+1)—O—SO₃Na,where n=12 to 18.

The table below shows the ascertained amounts of foam and methioninebulk densities as a function of type and concentration of the mixturesused as crystallization additives, the total active ingredient content(without water) being given.

Additive Concentration (ppm) Amount of foam (ml) Bulk density (g/l) None— 300 507 1 200 180 614 1 400 75 626 1 1000 10 587 1 1200 10 464 1 200010 410 1 4000 5 356 2 200 40 613 2 400 5 633 2 1000 0 610 2 1200 0 651 22000 0 625 2 4000 0 639

It is observed that the crystallization additive according to theinvention at a low concentration improves the bulk density aseffectively as the additive according to EP 1 451 139 A1 and that theadditive according to the invention, in contrast to the additiveaccording to EP 1 451 139 A1, retains its effectiveness even at a highconcentration.

Example 2 Recrystallization in the Presence of Pure Antifoam, PureCrystallization Additives, and Mixtures of Antifoam and CrystallizationAdditive

Recrystallization experiments according to the procedure from Example 1were carried out with the addition of pure crystallization additivesaccording to the invention, of mixtures of the crystallization additiveswith the antifoam and of the pure antifoam. The table below shows theamounts of foam and methionine bulk densities ascertained here.

The pure antifoam (Comparative Example 1) was used in the form of anaqueous mixture consisting of 6.1% by weight of silicone oil with akinematic viscosity of 1000 mm²/s (AK 1000, Wacker-Chemie GmbH), 0.25%by weight of hydrophobicized silica (Sipernat D10, Evonik Degussa GmbH),2.6% by weight of a polyethoxylated fatty acid mixture (Intrasol® FS18/90/7, Ashland Deutschland GmbH), 3.7% by weight of a polyethoxylatedfatty alcohol mixture (2.35% by weight of Marlipal®, Sasol Germany GmbH,1.35% by weight of Brij C2, Croda Chemicals Europe).

The pure crystallization additives used were the following anionicsurfactants:

2) C_(n)H_(2n+1)—O—SO₃Na, where n=12 to 18 (Sulfopon® 1218G,Oleochemicals)

3) C_(n)H_(2n+1)—O—C₂H₄—SO₃Na, where n=8 to 18 (Hostapon® SCI 85,Clariant)

4) C_(n)H_(2n+1)—(OC₂H₄)₂—O—SO₃Na, where n=12 (Disponil® FES 27, Cognis)

5) C_(n)H_(2n+1)—(OC₂H₄)₁₂—O—SO₃Na, where n=12 (Disponil® FES 993,Cognis)

Comparative Example 6) C_(n)H_(2n+1)—(OC₂H₄)₃₀—O—SO₃Na, where n=12(Disponil® FES 77, Cognis)

For the mixtures of the antifoam with the crystallization additives, ineach case 5.1% by weight of the corresponding crystallization additivewas added to the aforementioned mixture and the water fraction wasreduced by 5.1% by weight of:

7) (1)+(2)

8) (1)+(3)

9) (1)+(4)

10) (1)+(5)

Comparative Example 11) (1)+(6)

Concentration Amount of Bulk density Additive (ppm) foam (ml) (g/l)Comparative 400 70 474 Example 1 2 400 30-40 537 3 400 160 564 4400 >300 560 5 400 >300 558 Comparative 400 >400 528 Example 6 7 400 5633 8 400 5 624 9 400 20-30 613 10 400 40 581 Comparative 400 60 548Example 11 

The results show that the pure antifoam does not result in animprovement in bulk density (entry 1). The crystallization additives 2to 5 according to the invention improve the bulk density to values>500g/l, but in the majority of cases bring about increased foaming. Thecombinations 7 to 9 according to the invention of antifoam andcrystallization additives lead to bulk densities>600 g/l, thecombination 10 according to the invention leads to bulk densities>500g/l, without increased foaming arising.

Example 3 Recrystallization in the Presence of Antifoam andCrystallization Additives or Antifoam and Mixtures of CrystallizationAdditives

Further recrystallization experiments according to the procedure fromExample 1 were carried out with mixtures of a antifoam and acrystallization additive or mixtures of an antifoam and severalcrystallization additives. For this purpose, the following mixtures wereused:

8) (1)+(3) in concentrations of 200, 400, 1200, 2000 and 4000 ppm

9) (1)+(4) in concentrations of 200, 400, 1000, 1200, 2000 and 4000 ppm

10) (1)+(5) in concentrations of 200, 400, 1000, 1200, 2000 and 4000 ppm

11) (1)+((3)+(2) at the ratio of 1:1) in concentrations of 200, 400,1200, 2000 and 4000 ppm

12) (1)+((4)+(2) at the ratio of 1:2) in concentrations of 200, 400,1200, 2000 and 4000 ppm

Additive Concentration (ppm) Amount of foam (ml) Bulk density (g/l) 8200 70 599 8 400 0 624 8 1200 0 616 8 2000 0 610 8 4000 0 610

Additive Concentration (ppm) Amount of foam (ml) Bulk density (g/l) 9200 60 598 9 400 40 607 9 1000 0 600 9 1200 0 612 9 2000 0 594 9 4000 0584

Additive Concentration (ppm) Amount of foam (ml) Bulk density (g/l) 10200 280 551 10 400 40 581 10 1000 20 579 10 1200 5 544 10 2000 5 545 104000 5 531

Additive Concentration (ppm) Amount of foam (ml) Bulk density (g/l) 11200 40 628 11 400 0 640 11 1200 0 624 11 2000 0 614 11 4000 0 612

Additive Concentration (ppm) Amount of foam (ml) Bulk density (g/l) 12200 60 602 12 400 20 605 12 1200 0 628 12 2000 0 625 12 4000 0 621

The results summarized in the tables above show that—in contrast to theprocess described in EP 1 451 139 A1—an increase in the concentration ofthe tested additives does not lead to a decrease in bulk density or atleast not to a significant decrease in bulk density.

Comparative Example 1 Recrystallization in the Presence of AnionicSurfactants

Recrystallization experiments were carried out with the anionicsurfactants (13) sodium dodecylbenzenesulfonate and (14) dioctyl sodiumsulfosuccinate known from JP 46 019610 B. Here, the pure surfactantswere used in a concentration of 400 ppm each.

Additive Concentration (ppm) Amount of foam (ml) Bulk density (g/l) 13400 >400 348 14 400 0 446

The experimental data show that these surfactants lead to results whichare worse than the results for the surfactants tested in Example 2.

Example 4 Recrystallization in the Presence of Nonionic Surfactants

Recrystallization experiments according to the procedure from Example 1were carried out with the addition of nonionic surfactants. Thefollowing sorbitan based surfactants were used in the recrystallizationexperiments, where the surfactants were each used in a concentration of400 ppm.

15) Tego SMO V; sorbitan monooleate (PET10-084)

16) Tego STO V; sorbitan trioleate (PET10-086)

17) Tego SMS 60; polyethoxylated sorbitan monostearate (Pet 10-087)

18) Tego SMS; sorbitan monostearate (Pet 10-088)

19) Span 60; sorbitan monostearate (Pet 10-095)

20) Span 80; sorbitan monooleate (Pet10-096)

21) Span 83; sorbitan sesquioleate (Pet10-097)

22) Span 65; sorbitan tristearate (Pet12-167)

23) Tween 61; polyethoxylated (4 EO) sorbitan tristearate (Pet12-169)

24) Tween 65; polyethoxylated (20 EO) sorbitan tristearate (Pet10-089)

Additive Concentration (ppm) Amount of foam (ml) Bulk density (g/l) 15400 0 356 16 400 0 483 17 400 320 526 18 400 0-5 346 19 400 0-5 345 20400 0 335 21 400 0 356 22 400 60 446 23 400 20 499 24 400 0 616

With the non-ionic surfactant polyethoxylated sorbitan monostearate(Tween™ 65 from Croda) in a concentration of 400 ppm a methionine bulkdensity of 616 g/l was achieved.

Example 5 Influence of the Potassium Ion Concentration on the BulkDensity of Methionine

1000 g of a 95° C.-hot solution of 100 g of methionine in 900 g of waterwere added dropwise, with stirring, to a 40° C.-warm suspension of 20 gof methionine in 180 g of water over 2 h, during which the temperatureof the initial charge of suspension was kept at 40° C. The experimentswere carried out in the presence of 400 ppm of total active ingredientcontent based on the total mass of the solution/suspension of a mixtureaccording to the invention of a crystallization additive and of anantifoam and of an amount of potassium hydrogen carbonate correspondingto the potassium ion concentration given in the table. The mixtureaccording to the invention of a crystallization additive and of anantifoam consisted of an aqueous solution of 6.1% by weight of siliconeoil with a kinematic viscosity of 1000 mm²/s (AK 1000, Wacker-ChemieGmbH), 0.25% by weight of hydrophobicized silica (Sipernat D10, EvonikDegussa GmbH), 2.6% by weight of a polyethoxylated fatty acid mixture(Intrasol® FS 18/90/7, Ashland Deutschland GmbH), 3.7% by weight of apolyethoxylated fatty alcohol mixture (2.35% by weight of Marlipal®,Sasol Germany GmbH, 1.35% by weight of Brij C2, Croda Chemicals Europe)and 5.1% by weight of a fatty alcohol sulphate (Sulfopon® 1218 G,Oleochemicals) according to the formula:C_(n)H_(2n+1)—O—SO₃Na,where n=12 to 18. The concentration of the pure crystallization additivewas 117 ppm.

The bulk density of the precipitated methionine was determined afterfiltration and drying.

K⁺ concentration (g/l) Bulk density (g/l) 0 160 2 560 4 590 8 570 10 57012 560 14 540

The addition of potassium ions accordingly leads to an improvement inthe bulk density even at a low concentration of the fatty alcoholsulphate used as crystallization additive.

The invention claimed is:
 1. A process for preparing D,L-methionine, theprocess comprising feeding carbon dioxide to an aqueous potassiummethioninate solution obtained by hydrolysis of5-(2-methylmercaptoethyl)hydantoin, in order to precipitate out a crudemethionine, which is separated off and purified, wherein: an aqueoussolution of the separated-off crude methionine is purified byrecrystallization from a solution comprising an antifoam, potassium ionsand a crystallization additive; the crystallization additive is anonionic or anionic surfactant, or a mixture of different nonionic oranionic surfactants; the recrystallization occurs by introducing a 60 to110° C.-hot methionine solution into a 35 to 80° C.-warm methioninesuspension, a temperature of which is lower than that of the introducedsolution, the temperature of the methionine suspension being maintainedbetween 35 and 80° C. during the addition; and the crystallizationadditive is one of the compounds shown in formulae 1 to 3, or a mixturethereof:R¹—O—SO₃M  (formula 1)R²—O—(CH₂)_(n)—SO₃M  (formula 2)R³—(OC₂H₄)_(n)—O—SO₃M  (formula 3); n is an integer from 1 to 12; M issodium or potassium; and R¹, R² and R³ are each independently a linear,branched or cyclic, saturated or unsaturated C₈ to C₂₀ alkyl group or anaryl group.
 2. The process according to claim 1, wherein n is 2 and R¹,R² and R³ are each independently a linear, saturated C₈ to C₁₈ alkylgroup.
 3. The process according to claim 1, wherein a concentration ofthe crystallization additive in the solution from which therecrystallization takes place is at least 50 ppm based on a total massof the solution, suspension, or both.
 4. The process according to claim1, wherein the antifoam comprises silicone oil.
 5. The process accordingto claim 1, wherein a potassium ion concentration in the solution fromwhich the recrystallization takes places is from 1 to 30 g/kg.
 6. Theprocess according to claim 1, wherein a potassium ion concentration inthe solution from which the recrystallization takes place is from 5 to10 g/kg.
 7. The process according to claim 1, wherein therecrystallization occurs by introducing an 85 to 110° C.-hot crudemethionine solution into a 35 to 60° C.-warm methionine suspension, suchthat a temperature of a resulting mixture is kept constant between 35and 60° C.
 8. The process according to claim 1, wherein therecrystallization occurs in two stages, such that: in a firstrecrystallization stage, an 85 to 110° C.-hot crude methionine solutionis introduced into a 60 to 80° C.-warm methionine suspension, and atemperature of a first resulting mixture is kept constant between 60 and80° C.; and in a second recrystallization stage, the 60 to 80° C.-warmmethionine suspension obtained in the first recrystallization stage isintroduced into a 35 to 60° C.-warm methionine suspension, such that atemperature of a second resulting mixture is kept constant between 35and 60° C.
 9. The process according to claim 1, wherein therecrystallization occurs by vacuum crystallization, such that a pressurein a first recrystallization stage is from 100 to 1000 mbar and, if atwo-stage recrystallization is carried out, a pressure in a secondrecrystallization stage is from 35 to 200 mbar.
 10. The processaccording to claim 1, wherein some of the methionine suspension isremoved from the first and/or one of the other recrystallization stagesand is returned again via a circulation circuit, wherein a temperatureof the suspension in the circulation circuit is increased by 6 to 12° C.